1. Field of the Invention
The present invention relates to a copier, printer, facsimile apparatus, multifunction apparatus or similar image forming apparatus configured to form a toner image on an image carrier with toner grains having a small size and then transfer the toner image to a sheet or recording medium.
2. Description of the Background Art
It is a common practice with an image forming apparatus to form a latent image on a photoconductive element or image carrier, which may be implemented as a drum or a belt, develop the latent image to thereby produce a corresponding toner image, and transfer the toner image to a sheet by use of image transferring means, which may also be implemented as a belt or a roller. A color copier, for example, includes four developing units each storing one of black toner, cyan toner, magenta toner and yellow toner. The developing devices respectively form a black, a cyan, a magenta and a yellow toner image on associated photoconductive drums. Such toner images of different colors are sequentially transferred to a sheet one above the other by image transferring means, which may be implemented as a belt or a roller, completing a full-color image on the sheet. Subsequently, a fixing unit fixes the full-color image on the sheet.
In the image forming apparatus, when an image transfer clutch is coupled in accordance with the rotation of a main motor, the clutch causes a cam to rotate. Consequently, before toner deposited on the drum in a developing zone passes a nip for image transfer, the belt or the roller for image transfer contacts the drum, so that the toner deposits on the belt or the drum. The toner deposited on the belt or the drum is transferred to the reverse side of a sheet (so-called offset) due to defective cleaning.
A current trend in the image forming apparatus art is toward the use of toner grains and carrier grains having a small size each. Toner grains with a small size enhance the resolution of an image and the reproducibility of thin lines. Also, carrier grains with a small size obtain a greater surface area relative to the carrier grains and thereby allow a toner content to be increased for improving developing efficiency. However, such toner grains and carrier grains bring about the following problems.
When the drum is in a halt or is rotating, the high toner content increases the amount of toner to be transferred from a developing sleeve to the non-image portion of the drum. If an image is formed in such a condition, then the toner is transferred from the drum to the belt or the roller for image transfer at the nip when the belt or the roller is brought into contact with the drum. Although a cleaning blade or cleaning means cleans the surface of the belt or the roller before a sheet arrives at the nip, the former is apt to fail to fully clean the latter when the toner with a small size is left on the belt or the roller in a great amount.
Further, because the timing for causing the belt to contact the drum is based on the rotation of the main motor, the belt contacts the drum before the toner deposited on the drum during the halt or the rotation of the drum passes the nip. As a result, the toner is transferred from the drum to the belt and again brings about offset ascribable to defective cleaning.
Japanese Patent Laid-Open Publication No. 8-297420, for example, discloses a system in which a potential of the same polarity as the toner is applied to the belt in order to promote efficient cleaning. This system, however, has a problem that the belt must make at least one turn before a sheet arrives at the nip in order to electrostatically return the toner deposited on the belt at the nip to the drum, extending a period of time up to the start of image transfer. Another problem is that the toner deposited on the belt cannot be entirely returned to the drum like the toner cannot be transferred from the drum to a sheet with 100% efficiency. Moreover, nip pressure causes part of the toner to be transferred from the drum to the belt in addition to the electrostatic force.
Japanese Patent Laid-Open Publication No. 11-38776, for example, proposes a cleaning device assigned to the belt and including second cleaning means in addition to the cleaning blade or similar cleaning means. The second cleaning means is implemented as a roller or a brush to which a voltage of the same polarity as the toner is applied. The second cleaning means, however, increases the cost of the cleaning means and makes the entire apparatus bulky and sophisticated.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Utility Model Publication No. 7-53091 and Japanese Patent Laid-Open Publication Nos. 9-90778, 2000-47475 and 2000-330327.
It is an object of the present invention to provide an image forming apparatus capable of preventing toner from being transferred from an image carrier to the reverse surface of a sheet by way of image transferring means to thereby insure high image quality.
It is another object of the present invention to provide a small size, simple, low cost image forming apparatus.
An image forming apparatus of the present invention includes a rotatable image carrier for forming a toner image thereon, an image transferring device for transferring the toner image to a recording medium, and a moving device for selectively moving the image transferring device toward or away from the image carrier. The moving device moves the image transferring device toward the image carrier on the elapse of at least a period of time necessary for the image carrier to rotate from a rotation start position to a reversely rotated position. Assume that a distance between a position where the image transferring device approaches the image carrier and a position where the toner image starts being formed on the image carrier is a, that the image carrier is reversely rotated by a distance of b, that the linear velocity of the image carrier is V, and that a period of time between the start of rotation of the image carrier and image formation is c, then a period of time t necessary for the image transferring device approaches the image carrier after the start of rotation of the image carrier satisfies a relation of (a+b)/V less than t less than c.